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Windy Craggy |
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British Columbia, Canada |
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Cu Co Au Ag Zn
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Super Porphyry Cu and Au
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The Windy Craggy massive sulphide Copper, Cobalt, Gold, Silver, Zinc deposit is located in the Atlin district of far northwest British Columbia, Canada (#Location: 59° 44' 09" N, 137° 44' 37" W).
It lies within the allochthonous Alexander terrane which comprises a thick succession of complexly deformed Proterozoic to Permian basinal and platformal carbonate and clastic rocks with a subordinate volcanic component. These rocks have been subject to relatively low grade metamorphism and are unconformably overlain by a Late Triassic succession of calcareous turbidites and a bimodal volcanic suite which host the Windy Craggy deposit.
The host Upper Triassic section at Windy Craggy comprises mafic submarine volcanics with variable amounts of interbedded calcareous, argillaceous, sedimentary rocks, and has been informally termed the "Tats Volcanic Complex" which is subdivided into:
i). Upper Tats - at least 1500 m thick and mainly composed of pillowed basalt;
ii). Middle Tats - approximately 2000 m thick - interbedded graphitic and calcareous argillites with pillowed and massive mafic amygdaloidal to pillowed flows, tuff, agglomerate and limestone. The Windy Craggy mineralisation is hosted by clastic sediments, and mafic flows and sills in the lower sections of this subdivision;
iii). Lower Tats - approximately 1000 m thick - predominantly a mafic sill intruding calcareous argillite;
iv). A unit composed of mainly calcargillite-calcareous and graphitic shale, argillite and limestone of unknown age; and
v). A limestone unit made up of grey limestone of possibly Silurian-Devonian age in faulted contact with the overlying units.
The massive sulphide bodies within the lower sections of the Middle Tats Group occur near the transition from predominantly clastic units to overlying volcanic rocks. The sedimentary host rocks at this transition comprise non-calcareous to calcareous argillites which are indistinctly to well laminated and are predominantly fine to very fine grained. The individual argillite bands vary in thickness from less than a metre to 40 m, averaging 10 to 15 m. The argillites contain predominantly minor, very fine to coarse-grained disseminated euhedral cubes of pyrite and/or fine-grained pyrrhotite.
The basaltic host rocks are fine grained and are commonly amygdaloidal. Less commonly, flows may be porphyritic and are usually pervasively chloritised and carbonatised and are generally only slightly foliated. Sills are conformable with bedding, medium grained and have a doleritic texture, while dykes crosscut all lithologies, including the massive sulphide bodies, are generally lighter in colour and finer grained than sills, ranging from less than 10 centimetres to several metres in width.
Medium to coarse grained dolerite bodies, which are 1 to 40 m thick and of limited extent, occur below the massive sulphides, in places, hosting stringer sulphides. They are moderately to strongly altered, containing calcite, chlorite and epidote and are geochemically similar to overlying footwall and hanging wall flows, most likely representing the feeder conduits to the overlying volcanics.
The massive sulphides and enclosing hosts have been subjected to two phases of deformation producing isoclinal and open folds respectively. The main faults close to the deposit strike are steeply dipping and strike NW, subparallel to the strike of the host rocks.
Continuous massive sulphide mineralisation is developed over a minimum strike length of 1600 m, at least 600 m vertical extent, and greater than 200 metres in width. It appears to consist of two discrete sulphide bodies, the North and South Sulphide Bodies, each with a variably developed stockwork/stringer zone, although a third has been indicated by drilling and it remains open both at depth and along strike. In addition, one or more smaller massive sulphide lenses have also been recognised.
The tabular to lenticular, concordant North Sulphide Body is about 120-150 metres thick by 500 metres in diameter. The body is elongated in a WNW direction and dips moderately to steeply to the NNE. The South Sulphide Body is more deformed, is lensoidal and plunges steeply to the SE, extending to the SE as a series of 15 to 60 m wide massive sulphide lenses.
A vertical zonation is recognised in both sulphide bodies, from the footwall stringer to massive pyrrhotite, to massive pyrrhotite-pyrite, to massive pyrite, to massive pyrite-calcite-sphalerite, to a cap of discontinuous chert-carbonate-sulphides. Within this zonation, three types of mineralised massive sulphide are recognised, namely: i). massive pyrrhotite with lesser chalcopyrite; ii). massive pyrite with lesser chalcopyrite; and iii). massive pyrrhotite and pyrite with lesser chalcopyrite and magnetite. The massive sulphides of the North Sulphide Body, which commonly display breccia textures, has a pyrrhotite-rich core grading to a pyrite-rich outer and upper sections.
A stringer zone is found beneath both sulphide bodies and comprises irregular sulphide veins within pervasively chlorite and silica altered wallrock. The stringer sulphides are predominantly pyrrhotite with lesser chalcopyrite and, in places, pyrite in a gangue of quartz, carbonate, chlorite and albite. The sulphide content and density of veining increase upwards.
The principal hypogene ore assemblage includes pyrrhotite, pyrite, chalcopyrite and magnetite, with lesser digenite, sphalerite and rare gold, electrum, marcasite and arsenopyrite. An upper 90 m thick supergene zone of approximately 2 Mt has also been identified comprising pyritic mud, chalcocite, cuprite, native copper, hematite and sphalerite.
Pyrite occurs in two forms: i). early, colloform to framboidal spheres and colloform layers; and ii). later, recrystallised euhedral, equant cubes and pyritohedrons intergrown in a boxwork fashion. Pyrrhotite, which is coeval with chalcopyrite, is predominantly later than the early pyrite. Marcasite, where present, occurs as minute, rounded blebs in pyrrhotite. Magnetite occurs as euhedral dodecahedrons and octahedrons included within pyrite, pyrrhotite, chalcopyrite, carbonate and rarely sphalerite, although in the South Sulphide Body, bifurcating veinlets of magnetite crosscut massive pyrrhotite+ pyrite+chalcopyrite. Sphalerite occurs as small, isolated, anhedral grains within carbonate and as narrow veinlets crosscutting other sulphides and is found interstitial to euhedral pyrite. Digenite occurs as irregular, anhedral blebs with chalcopyrite and sphalerite, with which it displays mutual boundaries interstitial to, and as overgrowths on pyrite. It is also found as inclusions within chalcopyrite. Arsenopyrite is exceedingly rare, occurring as crystals that have chalcopyrite and pyrrhotite overgrowths. Native gold and electrum occur as: i). blebs associated with chalcopyrite and pyrrhotite fracture fillings in pyrite within massive sulphides; ii). anhedral inclusions within recrystallized, euhedral pyrite in massive sulphides; and iii). as inclusions within, and intergrowths with calcite in the gold-enriched portions of chert-carbonate-sulphide bands. Bismuth telluride occurs as very small rounded blebs included in pyrrhotite and may be present as wehrlite, hedleyite or tellurobismuthinite.
Graphite occurs as very fine folia and disseminations deflected around sulphides, quartz and plagioclase grains with semimassive to massive sulphide, particularly in stockwork mineralisation developed in argillite. Quartz occurs as discrete grains interstitial to sulphides and commonly intergrown with carbonate. Hisingerite is present in anastomosing or horsetail veins and veinlets crosscutting massive sulphides in the South Sulphide Body. Stilpnomelane occurs as fibres intergrown with pyrrhotite, chalcopyrite, chlorite and quartz, and is also interstitial to sulphides within mineralization.
Measured geological reserves have been quoted at:
* 297 Mt @ 1.38% Cu, 0.2 g/t Au, 3.83 g/t Ag and 0.069% Co using a 0.5% Cu cut-off, to
* 198 Mt @ 1.75% Cu using a 1.0% Cu cut-off, or
* 139 Mt @ 1.96% Cu using a 1.5% Cu cut-off.
This summary is based on the more detailed online British Columbia Geological Survey MINFILE record summarising this deposit.
The most recent source geological information used to prepare this decription was dated: 2006.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Gammon J B, Chandler T E, 1985 - Exploration of the Windy Craggy massive sulphide deposit, British Columbia, Canada: in Nesbitt & Nichol (Eds), 1985 Geology in the real world - the Kingsley Dunham volume, Papers presented at the Dunham-Dunham Reunion, 14-17 April, 1985, The Inst. of Min. & Met. (London) pp 131-141
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MacIntyre D G, 1985 - The geochemistry of basalts hosting massive sulphide deposits, Alexander Terrane, Northwest British Columbia: in British Columbia Ministry of Energy, Mines and Petroleum Resources, Geological Fieldwork, 1985, Paper 1986-1 pp 197-210
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Taylor C D, Premo W R, Meier A L and Taggart J E, 2008 - The Metallogeny of Late Triassic Rifting of the Alexander Terrane in Southeastern Alaska and Northwestern British Columbia: in Econ. Geol. v103 pp 89-115
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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